The CO2 “fertilization” effect won’t deter climate change

The CO2 fertilization effect is not going to “save” us from the consequences of global warming. News coverage has focused almost solely on the “greening” angle of increased levels of atmospheric CO2 and neglects to mention negative impacts of climate change that are highly detrimental to human agriculture and plant ecosystems in general. Climate impacts like drought, floods, extreme weather, shifting seasons, and increasing ranges of weeds, invasive species, and plant pests will all negatively impact crop yields.

The following is a guest post by Climate Nexus. Full text in PDF format here.

The CO2 “Fertilization” Effect Won’t Deter Climate Change

A recent paper published in Geophysical Research Letters has found that increased levels of CO2 is leading to increased plant cover in some warm, arid environments. The study references a real trend, but news coverage has focused almost solely on the “greening” angle and neglected to mention the many negative impacts of climate change that are extremely detrimental to human agriculture and plant ecosystems in general. These impacts more than outweigh any positive “greening” influence of CO2.

At the most basic level, agriculture was developed for a stable climate, which climate change is rapidly destabilizing. Climate impacts like drought, floods, extreme weather, shifting seasons, and increasing ranges of weeds, invasive species and plant pests will all negatively impact crop yields.

Looking at increased CO2 as a welcome boost to greening is like being happy that your septic tank is broken because it will fertilize the lawn.

The claim:

Based on satellite observations, researchers found that leaf cover in some arid zones increased by 11% as a result of a process called the CO2 fertilization effect, which helps plants grow more efficiently in high-CO2 environments. Subsequent coverage of the study omitted most of the other known effects of CO2 and global warming, giving the impression that increasing CO2 will result in a lush, green planet and more productive agriculture.

The facts:

The CO2 fertilization effect is by no means going to “save” us from the devastating consequences of global warming.

Weeds and other undesirable plants experience CO2 fertilization as well. Many weeds are tropical or subtropical, and are likely to move northward to new areas as a response to warming temperatures. This topic is covered at length in the USDA report of the effects of climate change on agriculture.

So while plant production in some arid regions may benefit from higher CO2 concentrations, in many other regions nutrient limitation will prevent much greening. Furthermore, any benefits that do occur are erased by the many negative impacts associated with climate change that lead to net losses.

Straight from the scientists:

“In those cases where CO2 fertilizations works, it will not necessarily be a long term effect because a nutrient limitation can kick in. That certainly happened in the case of the first big CO2 fertilization experiment at the Duke forest in North Carolina. I believe it is likely to happen in large areas of the forested tropics because nutrient poor soils are widespread. Much of the Amazon, for example, is likely to be limited by phosphorus.”

-- Thomas Lovejoy, Biodiversity Chair at the Heinz Center for Science, Economics and the Environment at George Mason University

“Simply, at higher CO2, plants maintain narrower openings on leaf surfaces because they can achieve the same internal CO2 concentration (inside leaves at the site of photosynthesis) with their "windows" more closed. Thus less water transpires out of their leaves -- and hence water is saved.

As the authors point out, what occurs in arid zones may not match what occurs elsewhere, where other limits (light, nutrients, low temperatures) may be more important than lack of water. In those non-arid regions, water savings from rising CO2 does occur and leaf area index is higher, but perhaps not proportionally as much as in arid zones. And those other limitations (e.g. low nutrients) may mean that rising CO2 increases plant production by less in such areas than in places where water is the main limit.”